Immunohistochemical (OKT6 monoclonal antibody) and electron microscopic studies (presence of Birbeck granules) of cells recovered by bronchoalveolar lavage disclosed accumulation of small numbers of Langerhans' cells in lower respiratory tract of smoking individuals. Cells with these characteristics are extremely infrequent in bronchoalveolar lavage fluid of normal, nonsmoking individuals. Because of these findings it appears necessary to evaluate very carefully the number and the morphologic characteristics of Langerhans' cells in bronchoalveolar lavage fluid of patients suspected of having pulmonary histocytosis X.